Egwene's stated goal is to tie all channelers to the tower, do you not think she would at least start this process with those who already are supposed to be a part of it?

I surely can't influence the sequence of events that Jordan envisioned, but I do see more of a need for Cadsuane's character in the tower than I do with Rand, where she has already achieved her goal of laughter and tears.

In the Seanchan attack, the green ajah learned that despite their stated mission, they really aren't that effective at it. They need a leader that is experienced and non-politically motivated. Cadsuane's abrasiveness, her intelligence, her legendary image, her experience, and even her various ter'angreal would make her an ideal candidate for such a role.